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Faxing with DrayTek T.38

DrayTek Vigor VoIP Routers

The Basics of Faxing

When a fax machine digitises a page of data, it scans at 203 x 98 dpi (dots per inch). This means, a sheet of A4 will be turned into a digital image of approximately 1675 by 1151 dots - just under 2 million dots in total (assuming normal resolution). The fax machine then encodes that page with a compression system in order that the page is transmitted more quickly. Instead of two million dots (1 or 0 for black or white), run lengths are transmitted. Instead of a 1675 x 1151 grid, all lines are joined up so that it becomes a single run of two million dots, which line wraps when printed at the other end. The compression system can work like this :

Original Data   : 00000000001111111111111111110000000011111111111111

Compressed Data : 10*0 18*1 8*0 14*1

As you can see, the compressed data takes up much less space, so can be transmitted quicker. This is exactly why a blank or scarcely printed sheet transmits very quickly, compared to a sheet with a lot of detail or images. Note : All data is transmitted in binary so the decimal example and arithmetic above is for illustration only.

The modem inside a Group 3 fax machine sends a signal at up to 14400 bits per second and as a timed each bit, zero or one, must last a specific amount of time so that every second 14400 bits of data are transmitted - no more and no less (unless the modem is running at a lower speed, obviously). If the signal is interrupted or delayed some of those bits will be lost or incomprehensible. As all bits are sent sequentially and not numbered (their position on the page is not included in the data) when data is lost, the whole page thereafter can become unreadable. Although regular Group3 faxing has error detection, it does not have error correction - it cannot recover lost data. At 14400bps, you can calculate that 2 million bits would take 138 seconds to transmit uncompressed. Add in negotiation and packet overheads, that's about 3 minutes, so with compression you can see that a typical fax ought to take about a minute to transmit, which it does!

Why do we need T.38 ?

It is assumed that you already know what VoIP is, and how you can use it to place voice calls. A Group 3 fax machine is designed to send documents using a regular analogue phone line, that you would otherwise use for voice calls. With a DrayTek T.38 equipped VoIP device, you still use your regular fax machine (or a fax modem built-into a PC).

If you have a VoIP port into which you connect a phone, you can connect a fax machine instead. Analogue phone lines these days are of excellent quality, so faxing is accepted as being very reliable usually. The Internet, however, is not a 'guaranteed' network. There is delay (latency), packet loss and jitter. Whilst human speech contains a lot of reduncancy and inefficiency (i.e. you can understand someone even if line conditions aren't good) fax is efficient; every bit of data is essential so sending an analogue fax signal across the Internet is extremely unreliable.

What does T.38 do ?

A T.38 equipped VoIP device is 'fax aware'. The phone call to the remote VoIP device is dialled normally, but when the fax machine starts to transmit its analogue fax signal, the VoIP device to which it's connected decodes (demodulates) the analogue audio signal back into digital form locally, transmits it to the remote VoIP device, which then creates a new analogue signal for the receiving fax machine. In this way, the analogue signal is not transmitted across the Internet and therefore you remove the potential for the VoIP encoded analogue signal suffering from loss or delay.

T.38 includes two methods of error correction to recover from minor packet loss. The data stream is transmitted with redundant (duplicated) data and checksums (CRCs). This way, if the receiving gateway detects that a packet has been lost or corrupted, it can extract it from the redundant data. T.38 also uses FEC (forward error correction) appended to the end of each T.38 packet.

To use T.38, you must be transmitting between two T.38 equipped VoIP devices/gateways.

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